1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine which is applied to vehicles equipped with an electronic throttle control unit which electrically drives a throttle valve, called a drive by wire (DBW).
2. Description of Related Art
In engines such as automobile engines, a drive by wire (hereinafter referred to as a DBW) connecting an accelerator pedal (hereinafter referred to as an accel pedal) and a throttle valve together by an electrical signal has hitherto been developed. In a DBW such as this, the accel pedal and the throttle valve are not mechanically connected, and based on a variety of parameters in addition to an operational quantity of the accel pedal (accel opening angle), a virtual accel opening angle ("pseudo" accel opening angle) is set by a computer. Based on this, the throttle valve can be controlled, and the DBW is also referred to as an electronic throttle control unit.
Therefore, for example, during idling where the accel pedal has not been operated (i.e., the accel opening angle is less than a small predetermined value), idling engine speed can be controlled, while the throttle valve is adjusted with fine precision. Also, in accordance with the traveling state of the vehicle and the operating state of the engine, the accel opening angle (driver's operation) is corrected in order to set a pseudo accel opening angle. With the control of the throttle valve based on this, engine running with a feeling of smoothness is realizable.
On the other hand, a spark ignition type in-cylinder injection internal combustion engine (hereinafter referred to as an engine), which is an internal combustion engine that uses sparks by spark plugs to ignite fuel (generally, a gasoline engine) that injects fuel directly into cylinders, has been put to practical use in recent years. In such an engine, an enhancement in the engine fuel consumption performance and an enhancement in the output performance are compatible with each other by making use of the characteristic that fuel injection timing can be freely performed and also the formed state of an air-fuel mixture can be freely controlled.
In other words, in this spark ignition type in-cylinder injection engine, fuel is injected on the compression stroke, and with this, an operation in a state in which fuel is extremely lean (i.e., a super-lean combustion operation in which an air-fuel ratio is extremely higher than a stoichiometric air-fuel ratio) can be performed by stratified-charge combustion. The engine is provided with a super lean operating mode (compression stroke injection mode or lean compression operating mode) as the combustion form and can realize a considerable enhancement in the fuel consumption ratio.
Of course, the spark ignition type in-cylinder injection engine can also perform a premixed combustion operation in which fuel is injected primarily on the intake stroke. In this case, fuel is injected directly into the combustion chamber (cylinder), whereby the greater part of fuel injected at each combustion cycle can be burned with reliability within the combustion cycle. The engine, therefore, can also enhance the engine output.
A premixed combustion operation such as this can also set as combustion form a lean operating mode (lean intake operating mode) which performs operation in a fuel-leaned state which is not as lean as the super lean operating mode (i.e., in a state in which an air-fuel ratio is higher than a stoichiometric air-fuel ratio), a stoichiometric operating mode (stoichiometric feedback operating mode) which performs feedback control on the basis of O.sub.2 -sensor information so that an air-fuel ratio reaches a stoichiometric air-fuel ratio, and an enriched operating mode (open-loop operating mode) which performs operation in a fuel-enriched state (i.e., in a state in which an air-fuel ratio is lower than a stoichiometric air-fuel ratio).
In general, if a requested output to the engine is small, i.e., if the revolution speed of the engine is low and also the engine load is low, the lean compression operating mode will be selected in order to enhance fuel consumption. As the engine revolution speed and the engine load increase further, the lean intake operating mode, the stoichiometric operating mode, and the enriched operating mode are selected in the recited order.
Incidentally, in the case of the super lean combustion operating mode (lean compression operating mode), there is a need to supply more air to the combustion chamber in order to make an air-fuel ratio high. However, in this lean compression operating mode, since operation is performed when engine load is low, i.e., when the stepping-on quantity of the accel pedal (accel opening angle) is small, the opening angle of the throttle valve corresponding to the accel opening angle cannot satisfy a required air-fuel ratio.
Hence, a technique where an electronically controlled valve (air bypass valve) is arranged in an air bypass passage bypassing an intake-air passage equipped with the throttle valve has been developed. In this technique, when intake air is insufficient at the opening angle of the throttle valve corresponding to the accel opening angle, the air bypass valve is opened based on a required air volume, thereby performing air supply.
Incidentally, applying the aforementioned DBW to the above-mentioned spark ignition type in-cylinder injection engine is also considered. In other words, in the DBW, since the opening angle of the throttle valve can be controlled without corresponding to the accel opening angle, more air than a quantity corresponding to the accel opening angle can be supplied to the combustion chamber. Therefore, for example, in the lean compression operating mode of the spark ignition type incylinder injection engine, even if the accel opening angle is small, a necessary quantity of air can be supplied to the combustion chamber.
In the case where such a DBW is adopted, it is desirable to prepare a counter plan against an unlikely failure of the DBW as well.
For instance, to achieve such a counter plan several sets of sensors are provided in the DBW, such as accel position sensors (APSs) and throttle position sensors (TPSs), and a plurality of sets of actuators for driving a throttle valve.
That is, two sets of sensors and two sets of actuators (these will hereinafter be referred together to as two sets of control systems) are provided, and if one control system fails, the DBW will be controlled by the other of the control system. The DBW is provided with a fail safe system by providing such a duplex control system, whereby safety and durability of the DBW can be enhanced.
Incidentally, in the above-mentioned dual control system of the DBW, when one control system has failed, a warning lamp within the instrument panel, for example, is lit, thereby informing the driver of an abnormality in the sensor or urging the driver to repair.
However, the driver may continue to travel without noticing a warning such as this. Also, even if the driver is aware of a warning such as this, the driver may continue to travel based on the knowledge that the DBW is equipped with a fail safe system such as this, the DBW is controlled by the other normal control system and has no adverse effect on the traveling performance.
However, if the driver continues to travel with one control system failed and, thereafter, the other control system fails, there is a problem that usual traveling will be difficult.
Note that although Japanese Laid-Open Patent Publication No. SHO 64-92553 discloses a technique which can ensure vehicle safety when an accel pedal fails, the disclosed technique is not one which solves the above-mentioned problems.
In addition, in the case where all of the above-mentioned plurality of control systems have failed, if the failures of the control systems should not be detected for some reasons, unpredictable motion of the throttle valve may occur.
Hence, even in the case where all of a plurality of control systems fail and also these failures cannot be detected, there is a desired demand for some counter plan to be prepared for safety.